Experiments performed during the current year indicate electrotonic interaction between the bundle branches and ventricular muscle only across the distal low ohmic resistance Purkinje fiber-muscle junctions. These junctions on the right side are anatomically remote from the proximal right bundle branch, electrotonically effecting no change in the proximal right bundle branch action potential. This circumstance probably accounts for the longer right bundle action potential and refractory period duration, and thus the propensity for functional right bundle branch block of critically timed premature supraventricular systoles. In the coming year experiments are planned that will hopefully elucidate the significance of electrical interactions between subendocardial Purkinje fibers and adjacent muscle. One series of studies will pertain to the Purkinje fiber-muscle junctions located in the basal portion of the left interventricular septum. There is evidence suggesting that the ohmic resistance of these junctions may be higher than at other more distal junctions. Other experiments will test the significance of subthreshold electrical interactions between peripheral subendocardial Purkinje fibers and ventricular muscle both in the presence and absence of specialized junctions between the two tissues. BIBLIOGRAPHIC REFERENCES: Lathrop, D.A. and Bailey, J.C.: The high resistance junction between the canine proximal left bundle branch and adjacent ventricular septal muscle. Clinical Research 23:IV-472A, 1975. (abstract). Lathrop, D.A., Elharrar, V., Foster, P.R., and Bailey, J.C.: Suppression of barium induced automatically with manganese and verapamil. Clinical Research 23:IV-472, 1975. (abstract).